Magnetoplasma microwave switch



1968 c. MOSKOWITZ 3,408,601

MAGNETOPLASMA MI CROWAVE SWITCH Filed June 7, 1967 been provided using semiconductor United States Patent M ABSTRACT OF THE DISCLOSURE A microwave switch, or attenuator, utilizing so-called anomalous skin effect is described herein. A high density electron plasma exists in most single crystal metals. The electron plasma in metals may be thought of as an electron gas.

In the presence of a in circular orbital paths, the planes of which are perpenfield. When the cyclotron frequency equals the microwave signal frequency, the electrons will absorb microwave energy at the same frequency. The magnetic field is adjusted to make the cyclotron frequency equal to the operating frequency of the wave guide so that the orbiting electrons will absorb the microwave energy and constitute an effective open switch. An electric biasing field applied to the single crystal metal body in a direction parallel to the magnetic field will increase the collision frequency of the electrons. A bias voltage that makes the collision frequency equal to or greater than the cyclotron resonance frequency causes the orbital movement of the electrons to cease and the anomalous skin effect is-destroyed, thus producing the effect of a closed microwave switch so that the microwave energy passes down the guide. Switching action is obtained by switching the biasing field on and off.

This application is a continuation-in-part of application Ser. No. 447,194, filed Apr. 12, 1965, which is a continuation-in-part application of Ser. No; 197,661, filed May 25, 1962, now abandoned.

This invention relates to microwave apparatus and, more particularly, to a new type of microwave switch which is capable of handling high microwave powers, in the ever expanding millimeter as well as in the three centimeter microwave range, at a very :fast rate.

Heretofore, numerous types of microwave switching devices have been used in wave guide apparatus. These switching devices may be divided broadly into two main categories: power reflective and power absorptive devices.

The present invention relates primarily to the power absorptive type.

lized glass vanes and similar devices.

Other wave guide switches have been provided using ferrite materials. Still other microwave switches have devices. Although the latter types of such devices are usually very fast acting they are not capable of dissipating large amounts of power because of the problems of making good thermal contact with the limited volume junction. The ferrite devices are capable of handling large amounts of power, but they have the disadvantage of being slow acting.

Cyclotron resonance-is discussed in the following publications:

Theory of Cyclotron Resonance in Metals in Soviet Physics JET P vol. 5, No. 4, November 1957, pages 730-744, by M. Y. Az-bel and E. Kaner;

a (b) Experimental Investigation of Cyclotron Resonance in Metals, in Soviet Physics JE'I P, vol. 12, No. 1, January 1961, pages 58-63, by same authors as above;

(c) Cyclotron Resonance Experiments in Silicon and Germanium, in Physical Review, vol. 104, No. 3, November 1956, pages 637-644;

(d) Cyclotron Resonance in Tin and Copper, in Physical Review, vol. 103, No. 5, September 1956, pages 1582-1583; and

, (e) Cyclotron Resonance,

11, 1960, pages 261-400.

No publications or patents disclosing the utilization of cyclotron resonance in single crystal metals as an absorptive device for absorbing electromagnetic wave energy is known by applicant.

It is a primary object of this invention to provide a novel and improved microwave switch device which is both fast acting and capable of handling large amounts of microwave power.

In accordance with the present invention a microwave switch device is provided which utilizes the phenomenon known as anomalous skin effect which exists in most single crystal metals. By employing this phenomenon in a suitable arrangement in a wave guide it is possible to provide a microwave switching device having a very high power handling capability and at the same time being very fast acting.

Another object of the invention is to provide a novel microwave attenuating device which makes use of the electron plasma existing in metal single crystals. The metallic surface is used as one conducting boundary for the guided electromagnetic wave and also serves as the attenuating section when the proper conditions are applied.

Another object is to provide a novel microwave attenuating device in which a body of single crystal metal is utilized as a boundary of a microwave field for the purpose of absorbing the wave energy.

A further object is to provide a novel and improved microwave switch device wherein a body of single crystal metal forms at least one boundary of a wave transmitted by a wave guide structure, the body of metal being immersed in a static magnetic field so that its interaction with microwave energy can be controlled by applying a bias voltage to the single crystal metal.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages, will be best understood from the following description when read in connection with the accompanying drawings, in which:

'FIGURE 1 is an isometric view of a wave guide section to which is applied a microwave switch in accordance with the present invention;

FIG. 2 is a sectional view on line 11-11 of FIG. 1

in Solid State Physics, vol.

, looking in the direction of the arrows;

FIG. 3 is a greatly 2; and

FIG. 4 is a side elevational view, partially in section illustrating an arrangement for maintaining the single crystal metal body at a cryogenic temperature.

To facilitate a clear understanding of the invention it enlarged view of a portion of FIG.

' 'As' previously mentioned, resistive microwave power absorbing devices in the past have utilized resistive elements which have surfaces that are pervious tohigh frequency current. It is well known in the art that low frequency currents and direct currents are carried within the body of an electrical conductor. As the frequency increases the normal'skin effect causes the electron current to flow on the outer surface of the conductor thus increasing the resistance. As the frequency becomes higher and higher the current is carried upon the surface of the conductor. At microwave frequenciesthere 'is substantially no penetration of the conductor by the high frequency electric field; the conductor then serves merely to guide the electric and magnetic fields at these high frequencies. At such frequencies then there are substantially no losses in a conducting surface and instead the conducting surface serves as a reflecting element.

The present invention provides a combination of elements constituting novel means for utilizing a phenomena, known as anomalous skin effect, for selectively overcoming the normal skin effect in a body of single crystal metal which is placed inside a waveguide with its surface skin serving as a boundary for the wave so that the microwave energy of a selected frequency in the wave guide will be absorbed. This then, effectively, constitutes what is the equivalent of a switch in a conventional electric circuit because it attenuates microwave energy going past a section in the wave guide in which this means is located.

The means provided for accomplishing this result depends upon the existence of a high density electron plasma which may be thought of as an electron gas in metals. It is well known that the motion of an electron, which has a velocity component normal to an applied magnetic field, is a circular path, the plane of which is normal to the applied magnetic field. The rotational (or cyclotron) frequency 9 depends upon the electron charge, mass and the applied field strength.

If a microwave signal of frequency w, which is equal to the cyclotron frequency 9, is caused to impinge upon, that is, is coupled to the electron motion, the electron as a consequence of its state of motion will absorb energy from the incident microwaveradiation and will therefore, so to speak, have its temperature increased. The greater the number of electrons in the electron plasma the greater will be the power absorption.

The behavior of the constituents of an electron plasma in a single crystal metal is similar to the behavior of the free electron just described above. However, there are some important differences. The electrons of the electron plasma undergo collisions which interrupt their state of circular motion. The more important of these collision processes, as far as the present invention is concerned, are those taking place between the electrons and phonons, the lattice imperfections and the surface of the metal. The word lattice is used here in the usual sense as referring to the regular placement of the atoms of the metal in specified ordered crystalline positions. Obviously, if these collisions are too frequent the desired circular or cyclotron motion will be interrupted and there will'be little chance of observing any absorption of the microwave energy. One requirement of the material used is that the electron collision'frequency 1/ '1' shall be much less than the cyclotron frequency, 9; 'r is the mean time between collisions.

The region in which the interaction between the microwave field and the orbiting electrons takes place is in the skin of the metal. As discussed previously, the normal skin effect limits the depth of penetrations of the microwave fields into the surface of the metal to a depth called the skin depth. For typical metals and practical magnetic fields the depth of the penetration of the skin is much less than the radius of the orbit of the rotating electron. In the usual polycrystalline metals the cyclotron resonance phenomena is practically non-existent because of the col 4 lisions of the electrons with phonons, lattice imperfections and the surface of the metal.

In accordance with the present invention the anomalous skin effect permits the microwave energy to penetrate to a skin depth, 5, indicated in FIG. 3, wherein the microwave energy interactswith and is absorbed by the spiralling electronsin theelectron plasma in the single crystal metal under conditions hereinafter outlined.

The rotational frequency in radians per second of the spiralling electrons of the electron plasma in a plane perpendicular to the magnetic field may be expressed where e is the electron charge; H is the applied magnetic field strength; In is the mass of the electron; and, c is the velocity of light.

For the cyclotron resonance phenomena and the anomalous skin effect to be established it is necessary that the electron succeed in making a large number of spiralling rotations over the length of the mean free path.

L=vr (2) was the previous restriction or requirement on the times also implies the requirement or the mean free path L greatly exceeds the orbit radius.

In most materials, the mean free path time is temperature dependent such that 1" increases as the temperature is decreased. In practice one chooses the'microwave frequency to which is to be switched or controlled and adjusts the magnetic field H such that the cyclotron frequency is equal to the operating frequency,

Having established S2 through equation (6), the unequality (3) 'must then be satisfied. This may require the use of cryogenic temperatures in order to make 1 large enough to satisfy (3). This will also depend upon the material chosen, the ambient environment temperature and obviously upon the operating frequency to.

In FIG. 4 there is illustrated an arrangement for maintaining the body of single crystal metal 12 and the portion of the wave guide 30 at a temperature in the cryogenic range in order to make 1- large enough to satisfy (3). A U-shaped portion of the wave guide 30 extends into a conventional cryostat 31. The cryostat may be filled with liquid nitrogen or other desired coolant 32, to such a level that the body 12 is maintained at the desired low temperature.

A magnetic field, with its lines of flux parallel to the longitudinal axis of the body 12, as in FIG. 1, may be provided by a suitably energized winding 36 surrounding the cryostat. An electric field may be supplied through conductors 37 and 38 in a manner similar to that described in connection with FIG. 1.

For single crystal metals, the skin depth 5 will generally be much less than the orbital radius r. However, in order to have an interaction with RF. electric field, the electrons must enter the sample surface within the skin depth without however colliding with the surface. This means 6 l Ztan L gives the tolerable limits upon qt for which the efl'ect will still be observable. As will be shown later when the bias voltage is applied the radius of the electron orbit becomes larger, the collision frequency becomes greater and the anomalous skin efiect is destroyed. The single crystal metal then acquires normal skin effect like polycrystalline metals and the microwave energy no longer is absorbed by the single crystal metal.

Referring to the drawings, an embodiment of the invention chosen for the purpose of illustration comprises a section of wave guide 10 adapted to be energized from any suitable microwave generator 11. The waveguide section 10 is of the type adapted to propagate the dominant TE mode and has disposed along one sidewall thereof a body 12 of single crystal metal having properties defined above necessary to carry out the objectives of the invention. Bismuth, tin and copper of single crystalline structure are examples of the material that may be used. As illustrated in FIGS. 1 and 2 the body 12 of the material may be in the form of a strip inserted inside the wave guide in good thermal contact with one short side of the rectangular Wave guide. As can be seen from the drawings, the outer surface of the strip serves as one boundary ducting relation, or it may form the wall itself.

Cyclotron resonance in the single crystal metal is estabfrequency. The condition in the strip 12 when cyclotron resonance exists corresponds to open circuit or no go condition in the guide; the condition when there is no cyclotron resonance corresponds to closed circuit or go condition in the wave guide.

Any suitable source of direct current biasing potential, such as the battery 16, can be supplied through switch 17 to the terminal points 13 and 14 on the strip 12. The

be observed here is that the circuit lncluding the battery 16, the switch 17 and the strip 12, does not include any inductance and, therefore, a control signal having a fast rise time, such as that illustrated by the pulse type signal 18, can be used for alternately establishing and destroying the cyclotron resonance in strip 12. This is significant instead of the battery 16 and the switch 17, a suitable function generator could be used for supplying a switching pulse, such as at 18, causing fast switching action.

section 10. The winding 19 may be energized by any suitable source of direct current, such as a battery 21, through a suitable switch 22 and may be adjusted by means of a cycles per second).

The operation of this device is illustrated by reference to the drawings. The winding 19 is not shown in FIGS.

tion 10 and the strip of material 12.

The material of the strip 12 should have an electron collision time '1', such that where ,u is the permeability and a the conductivity. For most metals 6 is On the order of one micron at 10 ghz. Thus, the strip assumes a frequency dependent absorpthe walls of the wave guide.

It is thus seen that the single crystal metal strip 12 becomes frequency dependent absorptive when it is imterminates the ergy and effectively wave guide, the wave guide thereby constituting a microwave switch.

It is to be understood that the invention is not limited in its use to a rectangular wave guide operatingin the TB mode. In general, the invention-is applicable to any wave guide structure in which the electric field is parallel to the inner walls of the guide. For example, in a circular wave guide operating in the TE mode a body of material corresponding to the strip 12 might be a longitudinal strip in the form of a section of a cylinder in contact with the walls of the guide.

I claim as my invention:

1. Electromagnetic wave switching apparatus comprising an electromagnetic wave transmitting line structure, a body of single crystal metal having electrons whose orbital radii are much less than their mean free paths, said body being supported by said structure, said body having an inner wall facing the axis of said structure, said inner wall being parallel to the axis of said structure, and serving as one boundary for wave energy in said structure, means for applying to said body of metal a magnetic field parallel to the axis of said structure and means for applying a biasing potential drop in said body along the length of said body for controlling the propagation of electromagnetic wave energy in said structure.

2. The combination as set forth in claim 1, in which said body has a length equal to several wavelengths of the wave propagated in said structure.

3. The combination as set forth in claim 1, in which said structure is a rectangular wave guide having such 3 dimensions as to support the dominant TE mode.

4. The combination as set forth in claim 3, in which said body oi metal has a wavelength equal to several wavelengths of the wave being propagated.

5. The combination as set forth in claim '3, in which saidbody of metal is in good thermal contact with a narrow wall of said structure.-

6. The combination as set forth in claim 1, in which said structure has first and second conductive boundaries for propagating electromagnetic waves having a magnetic field pattern forming closed loops parallel to said boundaries.

References Cited Lax, B., and Mavroides, J. G., Cyclotron Resonance, in Solid State Physics, vol. 11, 1960, pp. 261-400 (note especially articles pp. 312-331, and pp. 331-343).

Dexter, 'R. N., Zeiger, H. 1., and Lax, 'B., Cyclotron Resonance Experiments in Silicon and Germanium, in Phys cal Review, vol. 104, No. 3, November 1965, pp. 637-644.

Fawcett, E., Cyclotron Resonance i'n Tin and Copper," in Physical Review, vol. 103, No. 5, September 1956, pp. 1582-1583.

Azbel, M. Y., and Kaner, E. A., Theory of Cyclotron Resonance in Metals, in Soviet Physics JETP, vol. 5, No. 4, November 1967, pp. 730-744.

Azbcl, M. Y., and Kaner, E. A., vestigation of Cyclotron Resonance in Physics JETP, vol. 12, No. 1, January Experimental In- Metals, in Soviet 1961, pp. 58-63.

ELI LLEBERMAN, Primaiy Examiner.

L. ALLAHUT, Assistant Examiner. 

